The authors present the case of a 17–year–old male kidney transplant recipient who presented initially with dermatologic symptoms and was found to have histologic changes in the skin that were consistent with mycosis fungoides. Physical examination revealed no lymphadenopathy or hepatosplenomegaly, but an open lung biopsy revealed an Epstein–Barr virus (EBV)–negative monomorphic T–cell posttransplant lymphoproliferative disorder (PTLD) with a concomitant EBV–positive B–cell PTLD involving the same lesion of the lung. To the authors' knowledge, this is the first reported case of a PTLD presenting in the skin in which there was a subsequent discovery of composite, bilineal B– and T–cell PTLD of the lung.